JP2516292B2 - Atomic force microscope - Google Patents

Abstract

An atomic force microscope includes a tip (12) mounted on a micromachined cantilever (10). As the tip scans a surface (24) to be investigated, interatomic forces between the tip and the surface induce displacement of the tip. A laser beam is transmitted to and reflected from the cantilever for measuring the cantilever orientation. In a preferred embodiment the laser beam has an elliptical shape. The reflected laser beam is detected with a position-sensitive detector (22), preferably a bicell. The output of the bicell is provided to a computer for processing of the data for providing a topographical image of the surface with atomic resolution. <IMAGE>

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to atomic force microscopy, and more particularly to atomic force microscopy using microfabricated cantilever beams to achieve atomic resolution. In addition, atomic force microscopes can operate in vacuum, air, or liquid atmospheres to perform large surface area scans and provide common mode rejection for improved operation.

[0002]

Atomic force microscopy is based on the principle of detecting the force between a sharp stylus or tip and the surface under investigation. Atomic forces include displacement of a stylus attached to the end of a cantilever beam.
In its first embodiment, a tunnel junction was used to detect the movement of a stylus attached to a conductive cantilever beam. Later, optical interference was used to detect cantilever beam runout.

Bennig et al.
Review Letter (Phys. Rev. Lett.) ”, Vol. 5
6, No. 9, March 1986, pp. 930-933
A sharpened tip is attached to a spring-shaped cantilever beam, as described in, to scan the cross-sectional shape of the surface under investigation. The attractive or repulsive force created between the atoms at the apex of the tip and the atoms on the surface results in a small wobble of the cantilever beam. This shake is measured by a tunnel microscope. That is, the conductive tunnel type chip is arranged in the tunnel separated from the back surface of the cantilever beam, and the fluctuation of the tunnel current indicates the deflection of the beam. The force generated between the tip and the surface under investigation is
It is determined by the measured beam deflection and the characteristics of the cantilever beam.

G. McClelland et al., "Atomic Force Microscopy: General Principles and New Embodiments (At
omic Force Microscopy: General Principles and a Ne
w Implementation) '', Rev. Progr. Quart. Non-dest
r. Eval, vol. 6, 1987, p. 1307, and Y. Martin et al., "Atomic force micro-Force Mapping and Profiling on the Sub-100Å Scale".
scope-force mapping and profiling on a sub 100Å s
cale) ”, Journal of Applied Physics (J. Appl. Phys.), vo
l. 61, no. 10, May 15, 1987, pp.
Articles 4723-4729 describe using a laser interferometer to measure tip displacement. The advantages of optical detection over tunnel-type detection of cantilever beam deflection are increased reliability and ease of implementation, insensitivity to beam roughness, and low sensitivity to thermal drift.

[0005]

The atomic force microscope promises a future in the research and development and manufacturing environment because of its unique ability to image insulators and measure minute forces. To meet this promise, atomic force microscopes must be versatile. That is, it must be able to operate in a vacuum, air or liquid atmosphere. Moreover, it must be reliable, simple, compact, and economically designed. In addition, atomic resolution and the ability to scan large areas are additional requirements for some applications.

Therefore, the main object of the present invention is to detect the direction of the micromachined cantilever beam of an atomic force microscope by the deflection of the optical beam.

It is an object of the present invention to provide an atomic force microscope that uses an inertia mover coupled to a position sensitive detector.

Another object of the invention includes using an optical beam deflection technique and using a micromachined cantilever beam, and using an optical fiber to implement the optical beam deflection technique. And to provide a method of combining.

[0009]

According to the present invention, a piezoelectric tube is used to scan the surface and a micromachined cantilever beam is used to support the chip. Fine machining cantilever
The direction of the beam is detected by reflecting the laser beam on the back surface of the cantilever beam and detecting the reflected laser beam by a position sensitive detector, preferably a bicell. The laser beam source is preferably, but not necessarily, a single mode diode laser operating in the visible range. The laser output is coupled into a single mode optical fiber and the output of the optical fiber is focused on the back surface of the cantilever beam. In another embodiment, where the tip is supported by one or more arms extending from the end of the cantilever, the laser beam is focused on the arms in the area of the tip. Focusing on the back of the cantilever should be understood to include both focusing on the back of the cantilever beam itself, or on the arm in the area of the tip. The deflection angle of the reflected beam is detected by the bicelles. Common mode rejection of brightness variations is
This is achieved by arranging the bicells symmetrically.
In the present invention, placement is accomplished remotely by an inertia mover, as described below. Remote placement of bicells in an ultra high vacuum atmosphere is essential. Or, if the deviation from the center position in the bicell is small compared to the diameter of the laser beam, the common mode rejection is
For example, it can be achieved electronically by mounting a variable resistor in series with each segment of the bicell, equalizing the voltage drop across the resistor, and electronically centering the reflected laser beam at the face of the bicell. The output of the bicell is provided to a computer for processing the data and provides atomic resolution to the image of the surface under study.

The present invention relies on measuring the direction of the cantilever beam, not its displacement. The change in position translates into an angle change that is inversely proportional to the length of the cantilever. In the prior art atomic force microscope, the length of the cantilever beam was about 1 mm. The micromachined cantilever beam used in the present invention has a length of 100.
It is of the order of microns, which makes it possible to investigate the atomic resolution of the surface. When the present invention is carried out in an atmosphere that does not require a vacuum, the structure can be simplified. For example, the optical fiber can be omitted and the design can be made more compact. It also eliminates the need for inertia movers because the components of the microscope are accessible.

The output of the visible diode laser is preferably an elliptical beam with an aspect ratio in the range 5 to 7: 1. Such ellipses are generally considered unfavorable because they require optical modification, but it is advantageous to use an asymmetric beam shape when practicing the present invention. Because, by focusing the elliptical laser beam on the rectangular cantilever beam appropriately,
The sensitivity of laser beam deflection measurement can be increased and the alignment procedure can be simplified. An additional advantage of an elliptical beam is that a high power laser can be used, thereby achieving high measurement sensitivity, without exceeding the saturation limit of the bicell. It can also reduce the distance between the cantilever beam and the bicell, thus making the atomic force microscope more compact. If the beam is not essentially elliptical, such as the optical output of an optical fiber, then a cylindrical lens can be used to achieve an advantageous elliptical shape.

[0012]

EXAMPLES Atomic force microscopes are described, for example, in US Pat.
It is well known in the art, as described in 724318. This patent is incorporated herein by reference. While that patent describes a method of measuring the distance between a tip and a surface by monitoring tunneling current, the present invention measures the orientation of the tip by optical beam deflection, as described below. . The present invention is most advantageous for operation in inaccessible environments such as vacuum or ultra-high vacuum because it provides a remotely positionable position sensitive detector.

In the drawings, and in particular in FIG. 1, a schematic diagram of a cantilever beam deflection detection technique is shown. The stylus-cantilever system is located at the ends of a pair of support arms 14,
It includes a cantilever beam 10, for example made of silicon or silicon nitride, having a tip 12 in the range of 1 to 10 microns in length, preferably 5 micrometers. Alternatively, the tip 12 can be placed at the end of a single support arm extending from the end of the cantilever beam 10. The laser 18 transmits the laser beam through a lens 20, which is focused directly on the back surface of the arm 14 in the area of the chip.
In another embodiment (not shown) in which the tip extends directly from the cantilever beam, the laser beam is the back surface of the cantilever beam, or a reflector 16 attached to the cantilever beam to enhance the reflective properties. Is focused on. As used herein, the term "at the back of the cantilever beam" refers to the back of the cantilever beam itself, the back of a reflector coupled to the back of the cantilever beam, one or more of the areas of a chip. Interpreted as meaning the laser beam transmitted to the tip support arm. The laser beam is reflected to the position sensitive detector 22. The output of detector 22 is provided as one of the inputs of a general purpose computer. The x-axis and y-axis positions of the tip as it scans the surface of the workpiece are also provided as inputs to the computer, as is well known in the art. The computer then processes the data in a well-known manner to provide an image of the shape of the surface at atomic resolution. The images can be displayed on the screen or in a strip chart, can be in tabular form, or can be in visual form.

In the preferred embodiment, the laser beam 18 is a compact single mode diode laser which operates in the visible light spectrum, preferably 670 nm, for ease of matching. However,
Lasers operating in the infrared or ultraviolet range perform similar operations. The preferred position sensitive detector is a silicon bicell.

Generally, an atomic force microscope senses the motion of the tip back and forth with respect to the surface 24 to be examined. The motion of the tip is proportional to the interaction force between the tip and the surface of the workpiece w. However, according to the present invention, the orientation of the cantilever beam supporting the tip is measured. The measurements are performed in vacuum or ultra-high vacuum, in an aqueous atmosphere or in air, depending on the application. Typical atomic force microscope configurations for each atmosphere are well known to those skilled in the art.

FIG. 2 shows a modification of the well-known atomic force microscope which is most useful when the measurement is carried out in an ultrahigh vacuum atmosphere. However, even if the microscope is modified, in water,
It also operates in a non-vacuum atmosphere.

As shown in FIG. 2, the laser beam from laser 18 is coupled into a single mode optical fiber 26,
The output is focused by lens 28 onto a reflector 30 located behind the cantilever beam. The laser beam is reflected by the reflector to the position sensitive detector. For reference, the side of the cantilever beam containing the tip is referred to as the front side of the cantilever beam, and the side opposite the cantilever beam containing the reflector is referred to as the back side.

The piezoelectric tube 32 is used as a scanner. The micromachined cantilever beam has a length of 100 to 200 microns, preferably 100 microns and is 5 to 30 microns, preferably 20.
It has a micron width. The length and width dimensions are determined by the material of which the cantilever beam is made and have a stylus-cantileva system soft lever construction having a force constant of 0.01 to 100 Newtons / meter, preferably 0.1 N / m. Selected to achieve. The cantilever beam is coupled to the tube scanner 32. By using a microfabricated cantilever beam of small dimensions, imaging at atomic resolution, as opposed to the conventionally used cantilever beam, which is typically 1 millimeter long and limited in resolution. Is possible. The arrangement described above ensures a high scanning speed and gives almost no limitation on the size of the surface under investigation. The maximum scanning speed is typically 100 kHz due to the resonant frequency of the cantilever beam, and typically 10 kHz
It is determined by the resonant frequency of the tube scanner which is Hz.

The preferred position sensitive detector is a bicell, preferably a silicon bicell for detecting the angle of deflection of the laser beam. As shown in FIG. 3, the inertial mover ensures the removal of fluctuations in the intensity of the light striking the bicell by distantly locating the bicell with respect to the laser beam deflected by the cantilever beam.

The inertia mover includes a piezoelectric bar 36 whose length is varied by applying a sawtooth waveform voltage signal to the bar, as is well known in the art. A bicell 40 is disposed on the sapphire plate 38 near one end of the piezoelectric bar 36. In the position shown, the bicell is readily slidable in response to a suitable sawtooth waveform voltage signal applied to the piezoelectric bar 36 by a conductor (not shown), as is well known in the art.
The sapphire plate 38 and its associated bicell 40 move when a sawtooth voltage signal is applied to the bar. In this way, the position of the bicell 40 is set to 100.
It can be remotely controlled in steps as small as Angstrom. The inertia mover is compact and completely computer controllable, which is particularly advantageous when used in an ultra high vacuum atmosphere.

The use of scanners and detectors of the type described in FIGS. 2 and 3, ie, micron-sized micromachined cantilever and laser beam deflections, to orient the cantilever beam rather than its displacement. An atomic force microscope device for measuring is provided.
That is, a change in the position of the cantilever beam is translated into an angular change, which change is inversely proportional to the length of the cantilever beam, thus making small dimensions fully available. Another advantage of this atomic force microscope design is that all necessary alignments and adjustments exceed 10 microns, which is a range that can be achieved with simple and standard mechanical tools.

Depending on the application, it may or may not be necessary to operate in an inaccessible atmosphere, such as a vacuum atmosphere, which simplifies the above design. Since the components of the atomic force microscope can be accessed in liquid or gas, the optical fiber 26 can be omitted, and the design can be made more compact. When operating in a non-vacuum atmosphere, the inertia mover can be omitted. The main function of the inertia mover is to allow remote placement of the bicell in the vacuum chamber. For example, in-situ tip exchange can be incorporated into an atomic force microscope, but this feature results in a large misalignment of the optical path, requiring bicell relocation.

The output of the visible diode laser is an elliptical beam having an aspect ratio in the range of about 5 to 7: 1.
In the prior art, ellipses have been omitted by suitable optics. In contrast, asymmetric beam shape is an important aspect of other preferred embodiments of the present invention.

As shown in FIG. 4, an elliptical laser beam spot 42 is focused on a cantilever beam 44,
By making the major axis of the ellipse approximately parallel to the longitudinal axis of the cantilever beam, the elliptical beam spot is reflected by the cantilever beam according to the same aspect ratio and the resulting reflected spot size is the cantilever beam.・ Approximately 6 in the direction perpendicular to the longitudinal axis of the beam
It will be twice as small. The result is a geometry that adds the possibility of increasing the sensitivity of the beam deflection arrangement and provides a simplified alignment procedure. Furthermore, the size of the reflected laser beam 46 received by the bicell 40 is 5 to 7 times larger in the direction perpendicular to the deflection direction, as shown in FIG. 5, which is high without exceeding the bicell saturation limit. It is possible to use laser power and thus achieve high measurement sensitivity. Alternatively, the distance between the cantilever beam and the bicell can be reduced, thus leading to a more compact microscope.

[0025]

According to the present invention, the atomic force microscope operates in a vacuum atmosphere, an air atmosphere, or a liquid atmosphere,
Common areas to perform large surface area scans and improve operation.
Mode removal can be provided. Moreover, by using the elliptical optical beam, the measurement sensitivity was improved and the alignment procedure was simplified. This is because the elliptical light beam spot that hits the bicell, which is a position-sensitive detector, is made so that the movement direction of the spot due to the deflection of the cantilever crosses the major axis, so that even minute movements of the cantilever are sensitive. This is due to the effect that higher measurement sensitivity can be obtained since a laser with high sensitivity can be used. At the same time, by making the light beam striking the cantilever parallel to the longitudinal direction of the cantilever and the major axis of the light beam, more information about the deflection direction of the cantilever due to the reflected light is obtained, and the light spot is elliptical. By so doing, it is possible to simplify the operation of matching the respective elements and to obtain the effect of facilitating the matching.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a portion of an atomic force microscope.

FIG. 2 is a schematic view of a portion of the atomic force microscope that constitutes the present invention.

FIG. 3 is a schematic diagram of a preferred position sensitive detector that may be utilized in practicing the present invention.

FIG. 4 is a diagram of an elliptical laser beam spot focused on a micromachined cantilever beam forming part of an atomic force microscope.

FIG. 5: Elliptical laser received by a position sensitive detector
It is a figure of a beam spot.

[Explanation of symbols]

 10 Cantilever Arm 12 Chip 14 Support Arm 18 Laser 20 Lens 22 Position Sensitive Detector 26 Optical Fiber 28 Lens 30 Reflector 32 Piezoelectric Tube 36 Piezoelectric Bar 38 Sapphire Plate 40 Bicell

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Gerhard Mayer 2666, Farsands Court, Yorktown Heights, NY 10598, NY (56) References JP-A-59-43302 (JP, A) JP-A-SHO 61-133843 (JP, A) JP-A-62-156515 (JP, A) US Patent 4935634 (US, A) US Patent 4724318 (US, A) APPLIED PHYSICS L ETTERS. VOL. 55, NO. 25, 18 DECEMBER 1989 P.I. 2588-2590 APPLIED PHYSICS L ETTERS. VOL. 55, NO. 24, 11 DECEMBER 1989 P.I. 2491 -2493 REVIEW OF SCIENTI FIC INSTRUMENTS. VOL. 59, NO. 11, 1 NOVEMBE R 1988 P.I. 2337-2340 JOURRNAL OF APPLIED D PHYSICS. VOL. 61, NO. 10, 15 MAY 1987 P.I. 4723-4729

Claims (16)

(57) [Claims] 1. A tip, fixed to a deflectable cantilever, is placed sufficiently close to the surface of a workpiece to deflect it in response to a force generated between the tip and the workpiece. Applying the cantilever, detecting a light beam reflected from the cantilever with a position sensitive detector, and converting the reflected beam into an output signal indicating the deflection of the cantilever corresponding to the force, A method for inspecting the surface of a workpiece, characterized in that the light beam impinging on a position sensitive detector is an elliptical spot. 2. The method of claim 1, wherein the light beam is directed to the cantilever through an optical fiber. 3. The method of claim 1, wherein the light beam is a laser beam. 4. The method of claim 1, wherein the light beam is elliptical when striking the cantilever. 5. The method according to claim 4, wherein the spot of the elliptical light beam striking the cantilever has a major axis in a direction along the longitudinal direction of the cantilever. 6. The method of claim 1, wherein the spot of the light beam striking the detector moves transversely to the major axis of the light beam when the cantilever is deflected by the force. 7. The method of claim 1, wherein the spot of the elliptical light beam striking the detector has an aspect ratio of at least about 5: 1. 8. A chip fixed to a cantilever, comprising:
Generating a light beam that impinges on a tip and a tip that is placed in close proximity to the surface of the workpiece and at a distance close enough to cause the cantilever to deflect in response to the forces generated between the tip and the workpiece. Receiving the light beam reflected from the cantilever and the light beam source
Position detecting means for converting the reflected beam into an output signal indicating deflection of the cantilever corresponding to the force, wherein the light beam is elliptical when hitting the position detecting means. , Atomic force microscope. 9. The atomic force microscope of claim 8 further including means for providing relative scanning movement between the tip and the surface of the workpiece. 10. The atomic force microscope of claim 9, further comprising computer means coupled with said sensing means to generate a topographic image of the surface of said workpiece. 11. An optical fiber for striking the cantilever with the light beam from the light beam source,
The atomic force microscope according to claim 8. 12. The atomic force microscope according to claim 8, wherein the light beam source is a laser. 13. The atomic force microscope according to claim 8, wherein the optical beam is elliptical when hitting the cantilever. 14. The atomic force microscope according to claim 13, wherein the spot of the elliptical light beam striking the cantilever has a major axis in a direction along the longitudinal direction of the cantilever. 15. The atomic force microscope according to claim 8, wherein the elliptical light beam moves in a direction traversing the major axis of the ellipse when the force deflects the cantilever. 16. The method of claim 8 wherein the spot of the elliptical light beam striking the sensing means has an aspect ratio of at least about 5: 1.